When Ford designed the S550 driveline, they chose to use a ‘Guibo’ joint just ahead of the differential. Probably done for Noise/Vibration, it just adds another joint when adapting to our toys. Depending on whether the diff came from a manual or automatic vehicle, the joint was a different size, requiring a different adapter. Created a lot of confusion in the Factory Five world. Ford supplies the cast iron differential to the aftermarket and seems they may have changed the joint (for standardization?) and didn’t tell anyone. There still is a lot of confusion over this.
I came across a post on the FFCars forum (Super 8.8 Info) that seemed to address the issue. Searched all over and couldn’t come up with a confirmation. I finally tracked down Gerry and found out they run it on a GT4 Mustang that is raced on the West Coast. He said they had had issues with the joint not holding up and using an adapter, the bolts kept coming loose. He works in a Ford Service facility and was able to track down this part, a Sterling (Ford) pinion flange for their 10-1/4 / 10-1/2 differential.
Spoke to the guys at Denny’s and they confirmed the parts interchanged on paper but couldn’t say for sure if it worked. Went ahead and ordered the part and put it in. One more issue solved
Just in-case the links ever break, the part I used is a Ford #F8TZ-4851-B, from a 99-07 F350 w/ 10.5″ Sterling axle. Denny’s Driveshaft #9870041